Constant coefficient mechanical element



Patented Aug. 5, 1941 CONSTANT COEFFICIENT MECHANICAL ELEMENT James E.Harris, Newark, N. .L, assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York No Drawing. Application August27, 1938, Serial No. 227,064

7 Claims.

The present invention relates to ferrous alloys. More particularly itrelates to ferrous alloys in tended chiefly for use in making elementswhich operate through elastic distortion and which depend for theirusefulness upon the uniformity of their operation regardless of moderatetemperature change, such as springs and mechanical vibratory elementssuch as tuning forks. It also relates to vibratory or other elementsmade from these alloys. More particularly, the invention is concernedwith vibratory elements composed of alloys, the properties of which aresuch that a substantially constant frequency of vibration is maintaineddespite change in temperature; it is also concerned with springs made ofalloys which impart to the springs a substantial uniformity of operationdespite temperature change.

It is necessary that the frequency of tuning forks used for frequencystandards and for timing, as in athletic timing systems and picturetransmission systems, or those used for maintaining frequency on carriertelephone systems have a high degree of constancy. In carrier telephonesystems, for example, it is desirable that the change in frequency withtemperature be not greater than two or three parts per million perdegree centigrade. Two primary factors are involved in the change offrequency of a vibratory element with temperature. One factor is thechange in the mass distribution of the element due to the expansion orcontraction of the metal with temperature change. In the case of atuning fork or similar vibratory element which operates through flexure,such as a rod or reed, expansion of the metal causes an an increase intemperature usually cause an in- 1.

crease in frequency. In order to counteract the undesirablecharacteristics caused by a high coefficient of expansion, it has beenproposed to use invar, a nickel-iron alloy, having a coefficient ofexpansion which is substantially zero.

Although the use of invar overcame the disadvantages caused byexpansion, it tended to aggravate the change in frequency due to theother factor, change of modulus of elasticity. As the modulus ofelasticity increases, the frequency of vibration increases and as themodulus decreases, the frequency decreases. In most metals and alloysthe modulus varies greatly with temperature. In the case of a tuningfork or a similar vibratory element the condition desired is that thecoefficient of expansion and the methcient of modulus of elasticityshould be of opposite sign and have their values so related that theycompensate one another as nearly as possible, thereby producing aconstant frequency over a wide temperature range.

In an attempt to achieve this result, it has been proposed to addchromium to invar, thereby producing an element made up of an alloy ofiron, nickel and chromium.

According to the present invention this result is achieved to a highdegree and with advantages not obtainable with the chromium alloy byforming a vibratory element of an alloy containing principally iron,nickel and molybdenum in controlled proportions. By the use ofmolybdenum, the production of the alloy may be accurately controlled soas to produce consistently tuning forks whose coefficient offrequency ismaintained at a uniformly low value. One reason for this ease of controlis the purity of commercial molybdenum as compared with commercialchromium. Molybdenum can be purchased in a substantially and uniformlypure form whereas commercial chromium varies in purity from 97 to 100per cent. Further, tuning forks made of the molybdenum alloy are not sosensitive to a change in molybdenum content as the chromium alloys areto a change in chromium content. Thus a change of .1 per cent ofmolybdenum results in a change in the temperature coefficient offrequency of one part per million per degree centigrade, whereas achange of .1 per cent in the chromium content results in a change in thetemperature coefficient of frequency of from two to three parts permillion per degree centigrade. Therefore the inherent inaccuracies inproportioning are not so likely to produce a fork outside of thepermissible range of temperature coeflicient of frequency in the case ofthe molybdenum alloy as in the case of the chromium alloy.

Another advantage of the alloy of the present invention is the fact thatit is more readily machinable than are the prior used chromium alloys.

Furthermore, the molybdenum alloys produce elements, even those whichhave been heattreated, tend to age slightly causing a change The molybinfrequency over a period of time.

denum alloys possess this defect to a much lesser extent than do thechromium alloys.

Although the principal constituents of the alloy of the presentinvention are iron, nickel and molybdenum, small amounts of otheringredients such as carbon, manganese, cobalt, vanadium,

tungsten or other metals capable of modifyingthe properties of the alloywithout causing too great a change in the temperature coeflicient offrequency may be added. It is extremely desirable that small amounts ofcarbon be added since the presence of this element tends to render thealloy much more stable, thereby producing much more uniform results uponheat treatment. Since it is more desirable to work the alloy in formingthe vibratory elements rather than to use castings, it is usuallydesirable to add small amounts of manganese, since this element rendersthe alloy more easily workable. Where the vibration of the element is tobe maintained by a magnetic system, it is also desirable to add smallamounts of cobalt, since this ingredient raises the Curie point of thealloy. In the case of low temperature coefficient alloys the Curie pointis often very close to the normal operating temperature range of thetuning fork.

It is necessary that the constituents of the alloy be carefullyproportioned in order to produce a temperature coefficient of frequencyas close to zero as possible within the temperature range in which it isproposed to operate the vibratory element. A specific example of analloy which has been found to be preferable for tuning forks is thefollowing:

Per cent Iron 45.8 Nickel 34.8 Molybdenum 15.7 Manganese 2.2 Cobalt 1.2Carbon .3

These proportions may be varied somewhat within limits. However, since achange in the content of each constituent changes the coefficient offrequency, it is necessary that when the amount of one ingredient ischanged, the amount of one or more of the others also be changed, inorder to balance the effect of the first change.

Thus an increase of .1 per cent molybdenum decreases the temperaturecoefficient of frequency one part per million at room temperature, anincrease of .3 per cent nickel decreases the coefficient about two partsper million and an increase of .008 per cent carbon increases thecoefiicient about one part per million. With proper proportionin theconstituents may be varied within about the following limits:

Per cent Nickel 32 to 42 Molybdenum 13 to 18 Manganese to 3 Carbon 0 to.9 Iron Remainder perature coeflicient of frequency at the desiredoperating temperature.

Mechanical vibratory elements made from iron-nickel-molybdenum alloyshave been described above. Similar alloys having proportions within thelimits set forth above may also be used for making other elements, thecharacteristics of which are altered by expansion and by change inmodulus of elasticity. Among such elements may be mentioned balancesprings and springs for applying or measuring force. In each case,however, consideration must be given, in proportioning the constituentsof the alloy, to the different characteristics of the element impartedto it by its shape and the different uses to which the element is to beput.

Thus in the case of a spirally coiled balance spring, as contrasted witha tuning fork, expansion of the metal causes a decrease in frequency ofoscillation. Therefore, in order to compensate for expansion, thecomposition of the alloy must be so chosen that the temperaturecoefficient of modulus of elasticity is of the same sign as thetemperature coefficient of expansion. By carefully proportioning theconstituents of the alloy within about the limits set forth above thetwo chief factors affecting frequency can be made to compensate for oneanother so that the temperature coefficient of frequency at the desiredtemperature of operation is substantially zero. As in the case of thetuning fork or similar vibratory element, the principal ingredients ofthe alloy are iron, nickel and molybdenum but it is desirable to add oneor more of carbon, manganese or cobalt or other elements capable ofmodifying the properties of the alloy.

In the case of springs for measuring or applying force the change in thecharacteristics of the spring with expansion and with change in modulusof elasticity depends upon the type of spring, whether it is a leaf,spiral or helical spring, and upon the type of force applied, whether itis compressive, tensional or torsional. In each case the manner in whichthe characteristics change with temperature will be determinable by oneskilled in the art. The ingredients of the alloy can then be soproportioned within about the limits set forth above that thetemperature coefficient of modulus of elasticity will compensate thetem-- perature coefficient of expansion in such manner as to produce aspring which exhibits substantially no change in operation with moderatetemperature change.

In preparing the alloy it is desirable that oxidation or other chemicalreaction of the ingredients be prevented to avoid a substantialvariation in the proportions of the constituents and to avoid theintroduction of undesired compounds which would tend to alter thecharacteristics of the alloy. Therefore, the melting together of theconstituents is preferably. carried out in an inert atmosphere, such aspurified helium.

After the alloy has been cast, it may be subjected to hot rolling. Afterrolling, the billet is preferably annealed at about 950 C. and allowedto cool in the furnace. The annealing, by homogenizing the alloy andremoving strains introduced during rolling, renders the final productmore stable and the results more uniform. The billet may then bemachined to the desired form. After machining the article is preferablyagain annealed at about 950 C. and allowed to cool in the furnace toremove the strains introduced during machining.

It can be seen from the description above that the invention is of broadas well as specific application and is to be limited only by the scopeof the appended claims.

. What is claimed is:

l. A mechanical vibratory element made of an alloy comprising thefollowing ingredients in about the following proportions:

Per cent Iron 45.8 Nickel 34.8 Molybdenum 15.7 Manganese 22 Cobalt 1.2

Carbon .3

2. An alloy having about the following composition:

3. An element which operates through elastic distortion formed of analloy comprising about 32 per cent to about 42 per cent nickel, about 13per cent to about 18 per cent molybdenum and an alloy comprising about32 per cent to about 42 per cent nickel; about 13 per cent to about 18per cent molybdenum, a small amount of carbon less than about .9 percent, a small amount of manganese less than about 3 per cent, and theremainder substantially all iron.

5. A mechanical vibratory element made of an alloy comprising about 32per cent to about 42 per cent nickel, about 13 per cent to about 18 percent molybdenum and the remainder essentially iron.

6. A mechanical vibratory element made of an alloy comprising about 32per cent to about 42 per cent nickel, about 13 per cent to about 18 percent molybdenum, 0 per cent to about 3 percent manganese, 0 per cent toabout .9 per cent carbon and the remainder essentially iron.

7. A spring member made of an alloy comprising abdut 32 per cent toabout 42 per cent nickel, about 18 per cent to about 18 per centmolybdenum and the remainder essentially iron.

JAMES E. HARRIS.

